Conventionally, in an image forming apparatus based on a digital system, an image signal conversion table (look up table: described as "LUT" hereinafter) has been used to correct output characteristics of an output device (an image forming means) such as a printer or to emphasize a particular density area. This image forming apparatus generally comprises an image reading means, an image processing means, an image writing means, and an image forming means, and the LUT described above is incorporated in the image processing means, converts an input image signal inputted from the image reading means into the image processing means and outputs the converted signal as an output image signal to the image writing means.
On the other hand, the LUT is made reflecting output characteristics for image density of an image forming means such as a printer, so that, in a case where output characteristics of the printer has changed because of degradation or contamination of the image forming means or the like, the LUT can not play a role for calibration.
To overcome the defect, as one of controls called process controls executed inside an image forming apparatus, a plurality of patterns each having different image density are formed on an image carrier such as a photosensitive body or a transfer body; the patterns are detected by an optical sensor by checking the reflected light or transmitted light to change charged potential, development bias, or an exposure to a laser beam according to a result of detection, or to correct a gradation calibration table for gradation conversion for image data.
This calibrating method provides the merits that it enables automatic calibration within an image forming apparatus and that intervention by an operator is not required, but because of the characteristics of the optical sensor, there is no change in the side of high density where a quantity of deposited toner is large, so that calibration is possible only in a range from low density to intermediate density where a quantity of deposited toner is small. Further it is impossible to correct a quantity of toner which fluctuates according to change in a transfer capability of a transfer section associated with passage of time or to correct fluctuation of image density caused by change in fixing capability of a fixing section.
In contrast, there has also been proposed a calibrating method in which a pattern image formed on an image carrier and transferred and fixed on a transfer member is read with a scanner and a gradation calibration table is selected or prepared according to the read data, or color conversion coefficients and an RGB-YMCK color conversion table are prepared. In this method, different from the calibrating method using an optical sensor as described above, intervention by an operator such as mounting a discharged transfer member onto a document base is required, but calibration of a high image density section where a quantity of deposited toner is large is possible, and there is provided the merit that change of image density due to change of fixing capability in the fixing section can be calibrated. As the calibrating method as described above, there has been known, for instance, the invention disclosed in Japanese Patent Laid-Open Publication No. HEI 5-114962.
On the other hand, in a scanner used in an image forming apparatus like a color copying machine, because of change during passage of time in spectral sensitivity of an RGB filter in a CCD (Charge Coupled Device) constituting the scanner or because of difference of spectral sensitivity in each image forming apparatus, even if the same color patch pattern or a gradation pattern is read, a value read by each scanner may vary from unit to unit. Description is made below for this phenomenon with reference to FIG. 32 showing non-uniformity of spectral transmission characteristic of a B (Blue) filter in a CCD.
In FIG. 32, a) indicates a spectral transmission factor of a B filter 1 in a CCD, b) indicates a spectral transmission factor of a B filter 2 in the CCD, c) indicates a spectral transmission factor of yellow (Y) toner, and d) indicates a spectral transmission factor of black (K) toner in a case where a quantity of deposited toner is small. The horizontal axis indicates a wavelength, while the vertical axis indicates a spectral transmission factor or a spectral reflection factor of the CCD. In this figure, a) and b) show an example of non-uniformity in a spectral transmission factor of the B filter. Herein it is assumed that the spectral transmission factors a) and b) have been shifted by a rate indicated at h) respectively, but the same consideration is applicable also to a case where the assumption as described above is not made.
Namely, comparing the light transmitted through the B filter 1 in a) to the light transmitted through the B filter 2 in b) under the spectral reflection factor d) of black toner in a case where a quantity of deposited toner is small, a quantity of light having transmitted through the filter B1 is larger by a quantity of light having transmitted through a region e), but is smaller by the light having transmitted through regions f) and g) as compared to a quantity of light having transmitted through the filter B2. Herein the spectral characteristics in a) and b) have been shifted by a rate in h) respectively, in a case of the light having transmitted through the B filter 1 in a), the quantity of light having transmitted through the region e) is equal to the quantity of light blocked by the regions f) and g), and for this reason a difference for a Blue signal between a) and b) is small as far as the black toner is concerned.
To strictly examine the different above, it is necessary to take into considerations the spectral characteristics of the light source and dependency of sensitivity of a CCD on wavelength, but when calibrating shading of a scanner, by using an achromatic-colored reflection plate with low dependency of a spectral reflection factor for instance in gray on wavelength in a visible light area, the difference between a) and b) is calibrated.
However, in a case of yellow (Y) toner, the difference between filters in a) and b) appears as a difference of light having transmitted through or having been blocked by the region g), and the difference is clearly larger than that in a case of black toner. Also the difference can not be calibrated even by a shading calibration using an achromatic-colored reflection plate. The non-uniformity in spectral transmission factors among filters in a CCD can be calibrated in a case of achromatic colors like white or gray by means of shading calibration so that the RGB data become uniform, but in a case of a document with a spectral characteristic dependent on wavelength, the calibration can not be executed appropriately, and sometimes values for R, G, and B may vary unit by unit.
The difference generates some influences when reading transfer paper with a gradation pattern of each color YMCK or color patch recorded thereon with a scanner and preparing a gradation calibration table (.gamma.-calibration table) to correct gradation characteristics of a printer section from the read values (this operation is called Auto Color Calibration, and is described as ACC hereinafter), and offset from an idealistic state causes this phenomenon. Also in a case where the spectral transmission characteristic changes due to change of performance of a scanner in a CCD during passage of time, or in a case where the spectral reflection characteristics of YMCK toner being used changes, an RGB ratio in read values for the YMCK toner changes. As described above, if calibration is performed, after change of an RGB ratio in values read by a scanner for the YMCK toner, with an RGB ratio before the change, offset from a correct value becomes rather larger.